This invention relates to a method and apparatus for processing a workpiece in which a plasma struck in the chamber is stabilized during the transition between steps, particularly, although not exclusively, steps in a cyclic process in the treatment of the workpiece.
The plasma processing of a wafer or other workpiece may require certain plasma parameters to change between two or more states cyclically. The request for sudden changes in, for example, the gas pressure or discharge power between the states may lead to plasma instability or even lead to the plasma extinguishing.
A particular method to achieve highly anisotropic etches for high aspect ratio trenches is to use a switched process in which an etch step is alternated with a deposition step. Such a method is disclosed in WO-A-94/14187, EP-A-0822582 and EP-A-0822584. In the case of deep trench silicon etching, a passivating layer may be deposited on all surfaces of the wafer (including the trench), during the deposition step. During the initial part of the etch step, the passivating layer is required to be removed preferentially from the bottom of the trench by ion bombardment. This then allows the silicon to be removed by an essentially chemical process, from the bottom of the trench, during the remainder of the etch step. Alternating deposition and etch steps allows a high aspect ratio trench to be etched, contrasting with the use of the etch step alone which would result in a predominantly isotropic etch. One process gas or gas combination is switched off, or reduced in magnitude, and a second is switched on, or increased in magnitude, in changing from the etch step to the deposition step or vice versa during this cyclic process. During the change from one step to another step, plasma instability or extinguishing of the plasma may result.